Melt butter in a small bowl. In small saucepan, over high heat combine vinegar, wine, onion and tarragon. Boil approximately 1 minute to reduce liquid. Remove from heat and allow to cool to room temperature.

Meanwhile, fill bottom of double boiler with water and bring almost to a boil. Lower heat so that water is hot but not boiling. In top of double boiler, combine egg yolks and onion mixture. Place top of double boiler over bottom and gradually whisk in the melted butter in a slow, steady stream. Continue stirring with the whisk until sauce is thick. Adjust to taste with salt and pepper and serve immediately.